Methods have been described where acrylic acid (AA) is produced via the pyrolysis of polypropiolactone (PPL) (e.g., see U.S. Pat. No. 2,361,036). However, PPL pyrolysis as described in this and related literature does not produce acrylic acid of sufficient purity for direct use in radical polymerization for superabsorbent polymer (SAP) production. Instead, the methods require expensive and energy intensive purification of the acrylic acid before it can be polymerized to produce SAP. There is therefore a need in the art for methods of directly producing glacial acrylic without the need for expensive and energy intensive AA purification.
Glacial acrylic acid, a purified form of acrylic acid, can be used to make polyacrylic acid for superabsorbent polymers (SAPs). At least two problems currently known in the art hamper the production and/or purification of glacial acrylic acid.
First, acrylic acid is primarily produced via vapor phase oxidation of propylene via an acrolein aldehyde intermediate. Products of propylene oxidation, such as the acrolein aldehyde, and by-products of propylene oxidation, such as other aldehyde impurities, are difficult and expensive to remove from crude acrylic acid. Aldehyde impurities hinder polymerization to polyacrylic acid and discolor this polymer.
Second, acrylic acid is extremely reactive and susceptible to unwanted Michael addition and free-radical polymerization with itself. Therefore, even after glacial acrylic acid is purified, it gradually degrades unless stabilizers, such as radical polymerization inhibitors, are added to retard unwanted side reactions. Stabilizers, however, are expensive and may interfere with the conversion of acrylic acid to polyacrylic acid.
Thus, there is a need in the art for methods to produce acrylic acid, including glacial acrylic acid, on a commercial scale.